Photoconductive elements comprise a conducting support bearing a layer of a photoconductive material which is insulating in the dark but which becomes conductive upon exposure to radiation. A common technique for forming images with such elements is to uniformly electrostatically charge the surface of the element and then imagewise expose it to radiation. In areas where the photoconductive layer is irradiated, mobile charge carriers are generated which migrate to the surface of the element and there dissipate the surface charge. This leaves behind a charge pattern in nonirradiated areas, referred to as a latent electrostatic image. This latent electrostatic image can then be developed, either on the surface on which it is formed, or on another surface to which it has been transferred, by application of a liquid or dry developer composition which contains electroscopic marking particles. These particles are selectively attracted to and deposit in the charged areas or are repelled by the charged areas and selectively deposited in the uncharged areas. The pattern of marking particles can be fixed to the surface on which they are deposited or they can be transferred to another surface and fixed there.
Photoconductive elements can comprise a single active layer, containing the photoconductive material, or they can comprise multiple active layers. Elements with multiple active layers (sometimes referred to as multi-active elements) have at least one charge-generating layer and at least one charge-transport layer. The charge-generating layer responds to radiation by generating mobile charge carriers and the charge-transport layer facilitates migration of the charge carriers to the surface of the element, where they dissipate the uniform electrostatic charge in light-struck areas and form the latent electrostatic image.
The photoreceptor properties that determine the radiation necessary to form the latent image are the quantum efficiency, the thickness, the dielectric constant, and the existence of trapping. In the simplest case, where trapping can be neglected, the exposure can be expressed as: ##EQU1## where E is the exposure in ergs/cm.sup.2, .epsilon. the relative dielectric constant, L the thickness in cm, e the electronic charge in esu, .lambda. the wavelength in nm, .phi. the quantum efficiency, k a constant equal to 5.2.times.10.sup.-13, and .DELTA.V the voltage difference between the image and background area, V.sub.i -V.sub.b. The quantum efficiency, which cannot exceed unity, represents the fraction of incident photons that are absorbed and result in free electron-hole pairs.
For electrophotographic processes known heretofore, .DELTA.V is typically 400-500 V. Assuming typical values of .epsilon.=3.0, .lambda.=500 nm, and L=10.sup.-3 cm, the above equation predicts an exposure energy of 11.8 to 14.7 ergs/cm.sup.2. This assumes that there is no trapping and is based on the absorbed radiation. In practice, the radiation is not completely absorbed, and the exposure is correspondingly larger. Thus, most photoreceptors require exposures in the range of 20-100 ergs/cm.sup.2 to form an electrostatic image. These are equivalent to ASA ratings between 0.1 and 0.02. In contrast, the exposure required to form a latent image in conventional silver halide photography is in the range of 10.sup.-2 to 10.sup.-1 ergs/cm.sup.2, or less, and, accordingly, the radiation sensitivity of electrophotography is less than that of conventional silver halide photography by a factor of at least 10.sup.-3.
In addition to electrophotographic speed, an important parameter with regard to the use of a photoconductive insulating element in an electrophotographic process is the exposure latitude, or, as it is often called, the dynamic exposure range. The conventional method for characterizing the response of a photoreceptor is to plot the surface potential versus the logarithm of the exposing radiation for a given initial potential, V.sub.o. Since the logarithm of exposure represents the optical density of the image which is to be reproduced, the linear portion of the V-logE curve gives the range of optical density in which the image can be faithfully reproduced by the surface potential. This exposure range is usually described as the dynamic range. Images comprised of a range of optical densities in excess of the dynamic range cannot be accurately reproduced by the photoreceptor surface potential. For this reason, the photoreceptor dynamic range is a critical parameter in the electrophotographic process.
The usual method for evaluating the dynamic range is based on a technique employed in conventional photography. This technique involves the following steps:
(1) The surface potential in volts is plotted versus the logarithm of the exposing radiation for a given initial potential V.sub.o, to thereby provide a V-logE curve. PA0 (2) The derivative of the curve is then determined and plotted on the same exposure axis. The derivative is expressed in units of volts/logE and defined as the contrast, .gamma.. PA0 (3) The dynamic exposure range, in units of logE, is then defined as the ratio of the initial potential, V.sub.o, to the maximum contrast, .gamma..sub.max. Defined in this manner, the experimental values of the dynamic exposure range very closely approximate the range of optical densities that can be accurately reproduced by the photoreceptor surface potential.
The fundamental phenomenon that controls the maximum contrast is the field dependence of the quantum efficiency, , .phi. (E). In cases where the efficiency is weakly field dependent, the contrast is high and the dynamic range correspondingly low. Conversely, materials which have strongly field dependent quantum efficiencies are low contrast, high range materials.
Photoconductive insulating elements comprising one or more layers of doped hydrogenated amorphous silicon have many valuable properties which render them commercially attractive and are currently of widespread interest in the art, but they exhibit a rather high contrast and thus a rather narrow dynamic exposure range, typically a range of about 0.7 to about 0.8 logE. While values of this magnitude are usually sufficient for the reproduction of digital information (line copy, for example), they are not sufficient for continuous tone reproduction (pictorial information, for example). Thus, the art of electrophotography would be greatly benefitted by the development of a successful technique for extending the dynamic exposure range of this important new class of photoconductive elements.
The most useful process for the manufacture of photoconductive insulating elements comprising a layer of doped hydrogenated amorphous silicon is a process comprising plasma-induced dissociation of a gaseous mixture of a silane (for example SiH.sub.4) and a doping agent, such as phosphine gas (PH.sub.3) or diborane gas (B.sub.2 H.sub.6). This process is carried out at elevated temperatures and reduced pressures, typically at temperatures of above 200.degree. C. and pressures of about one Torr.
It is toward the objective of providing an improved plasma-induced dissociation process for the manufacture of photoconductive insulating elements comprising a layer of doped hydrogenated amorphous silicon, whereby an extended dynamic exposure range is achieved, that the present invention is directed.